which shop did you intend to go to?
I also intend to go to Taipei for my wedding photos later this year.
It's true that they cannot mail you if your OP is bigger than 20x24,
because they only use surface mail. Not the Fed-Ex or DHL.
You may try an alternative;
Request them to have a bigger OP for you...of course the bigger the OP is, the better it is...then ask them to use DHL. But you have to pay for the cost.
I had asked about sending an OP which measures 60' big and it cost around NT$4000++ excluding insurance which is about NT$175 per NT$10000 coverage.
In HK, if you had asked around,
a 36x40 OP with frame cost around $3500 onwards, even if you go to Kam-Do.
Hope this may serve useful to you.
Mind if I ask what type of package did they offer to you?
When are you going?